The invention relates to the technology of coatings. In particular, the invention relates to coating materials which are hydrophobic, and to methods for making coatings from those materials.
Hydrophobic coatings are water-proof coatings which have immediate uses in reducing icing and fouling of other surface. Such coatings can also render protected surfaces resistant to attachment by water soluble electrolytes such as acids and alkalies, and by microorganisms.
In the past, surfaces have been protected against encrustation, corrosion, icing and fouling by means of coatings containing polymer films, hydrophobic solid fillers and hydrophobic liquids. One disadvantage of the use of such coatings is that they do not achieve multi-purpose protection since they are not generally versatile enough to protect against damage from a variety of causes.
It is well understood that the wettability of various materials is dependent on both the physical and chemical heterogeneity of the material. The notion of using the contact angle xcex8 made by a droplet of liquid on a surface of a solid substrate as a quantitative measure of the wetting ability of the particular solid has also long been well understood. If the liquid spreads completely across the surface and forms a film, the contact angle xcex8 is 0xc2x0. If there is any degree of beading of the liquid on the surface of the substrate, the surface is considered to be non-wetting. For water, the substrate surface is usually considered to be hydrophobic if the contact angle is greater than 90xc2x0.
Examples of materials on which liquid droplets have high contact angles include water on paraffin, in which there is a contact angle of about 107xc2x0. Many applications require a hydrophobic coating with a high contact angle of at least 150xc2x0, and preferably at least 165xc2x0.
A xe2x80x9cgelxe2x80x9d is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. The liquid prevents the solid from collapsing, and the solid prevents the liquid from escaping. The solid skeleton can be formed by linking colloidal particles together.
The present inventors have now developed methods for producing materials which, when coated on a surface, render that surface hydrophobic.
In a first aspect, the present invention provides a method of forming a material capable of being applied to a surface, the method including the steps of:
(a) providing precursors capable of reacting to form a gel;
(b) reacting the precursors together to form the gel;
(c) adding a particulate material to the gel to form a mixture, the particulate material being capable of chemically bonding with the gel; and
(d) treating the mixture such that a modified gel is formed in which the particulate material is bound to the gel, and the modified gel is capable of forming a surface which is chemically hydrophobic and has a surface roughness which physically enhances the surface hydrophobicity, such that water has a contact angle on the surface of at least 150xc2x0.
In a second aspect, the present invention provides a method of forming a coating on a substrate, the method including steps (a) to (d) of the first aspect of the present invention, and further including the steps of:
(e) applying the modified gel to the substrate; and
(f) treating the applied modified gel such that a coating is formed on the substrate, the coating having a surface which is chemically hydrophobic and has a surface roughness which physically enhances the surface hydrophobicity, such that water forms a contact angle of at least 150xc2x0.
Preferably, the hydrophobic surface defined in either the first or the second aspect of the present invention is such that water forms a contact angle of at least 155xc2x0 on it. More preferably, the contact angle is at least 160xc2x0. Even more preferably, the contact angle is at least 165xc2x0.
The hydrophobicity of the hydrophobic material when applied to a surface is preferably due to both the chemical properties of the modified gel and physical roughness of the material. It is envisaged that the modified gel of the first aspect of the present invention could be used to make solid materials in a range of possible forms, including bulk materials, thick coatings, and thin films.
The gel functions as a cross-linking agent which binds the particulate material, and attaches the modified gel to the substrate if required. Any known process for forming a gel may be used. Typically, the precursors defined in step (a) of the first and second aspects of the present invention at least include water, a solvent, and a metal alkoxide such as one of the following:
tetramethoxysilane (abbreviated TMOS), Si(OCH3)4 
tetraethoxysilane (abbreviated TEOS), Si(OCH2CH3)4;
titanium tetraisopropoxide, Ti(O-iso-C3H7)4;
titanium tetramethoxide, Ti(OCH3)4;
titanium tetraethoxide, Ti(OC2H5)4;
titanium tetrabutoxide, Ti[O(CH2)3CH3]4;
zirconium n-butoxide, Zr(O-n-C4H9)4.
The solvent may comprise an alcohol such as methanol, ethanol, isopropanol, or butanol. Alternatively, the solvent may comprise hexane or diethyl either.
For example, silicate gels may be synthesised by hydrolysing an alkoxide dissolved in an alcohol with a mineral acid or base, or organic acid or base. The end product is a silicon dioxide network.
Step (b) of reacting the precursors together in the first and second aspects of the invention may be implemented by refluxing the precursors for an extended period, such as a period in the range from 4 hours to 24 hours.
The particulate material defined in step (c) of the first and second aspects of the invention may consist of particles having substantially equal diameters, or alternatively having a spectrum of diameters. Preferably, at least some of the particles have diameters within a range from 1 nm to 500 xcexcm. More preferably, the range is from 1 nm to 100 xcexcm. Still more preferably, the range is from 1 nm and 1 xcexcm. Still more preferably, the range is from 1 nm and 100 nm, and even more preferably the range is from 5 nm and 50 nm. In one embodiment, the particulate material consists of particles with diameters in a range from 1 nm to 500 xcexcm. In a further embodiment, the primary particle diameter of the particulate component is the range from 5 nm to 50 nm. In yet a further embodiment, the average particle size is in the range from 5 nm to 20 nm. In yet a further embodiment, the average particle size is about 15 nm.
Step (d) of the first and second aspects of the invention may be implemented by firstly thoroughly mixing the mixture, such as in an ultrasonic bath. Optionally, an alcohol such as isopropanol may be added to the mixture during this step to aid in dispersing the particulate material. Secondly, the mixture may be refluxed to cause chemical bonding between the particulate material and the gel.
Step (e) of applying the modified gel to a substrate may be carried out by any known technique of forming a coating from a liquid, such as spin coating, dip coating or spray coating.
Step (f) may involve drying the applied modified gel until a solid coating is formed. There may be solvents which need to be removed from the modified gel, and in such a case, the drying may include heating the applied modified gel to a temperature which is at least high enough to evaporate the solvents. It will be appreciated that the drying temperature will depend on the melting point of the substrate and the type of gel. The drying time for a particular application will usually depend on the temperature used, and to some extent on the thickness of the coating. In the case of silica coatings, it has been found that a heating temperature of in the range from 120xc2x0 C. to 400xc2x0 C. over a period of 10-30 minutes is suitable when the substrate is capable of withstanding such a temperature. Vacuum drying, or a combination of vacuum drying and heating, may be preferable when the substrate has a low melting point.
Elasticity and flexibility of the coating may be enhanced by adding a polymer component to the gel during step (c). Alternatively, the polymer component may be added to the gel either directly before or directly after step (c). The polymer component preferably bonds with the gel and particulate material during step (d), and is preferably either hydrophobic, or rendered hydrophobic by the reaction in step (d).
Where a polymer component is mixed into the gel, the method may further include a step prior to step (d) of adding a surface modifier to the gel for enhancing the intrinsic chemical hydrophobicity of the hydrophobic surface. The surface modifier may additionally enhance bonding between the gel and particulate material. The surface modifier may be a compound containing one or more condensation cure groups and one or more hydrophobic groups. The one or more condensation cure groups may include one or more of the following groups: acetoxy; enoxy; oxime; alkoxy; or amine. The surface modifier may comprise SiR(OAc)3, where R is a hydrophobic group such as methyl, ethyl, vinyl, or trifluoropropyl, and Ac is an acetyl group. In a preferred embodiment, the particulate material comprises silica, the gel comprises a silicate gel, and the surface modifier comprises methyltriacetoxysilane. The step of adding the surface modifier may take place either immediately before step (c), during step (c), or immediately after step (c).
The gel, the particulate material, and optionally the polymer component, preferably form a slurry when mixed and reacted together in step (d) of the first and second aspects of the invention.
In a preferred embodiment, the particulate material consists of flame-hydrolysed silica powder, and the gel precursors include a compound capable of forming a silicon dioxide gel, such as either TMOS or TEOS. A suitable polymer component in this case is polydimethylsiloxane (PDMS), a polymer with hydroxyl groups terminating the ends of each chain. The resultant modified gel consists of silica particles chemically bound to a siloxane network, and surrounded by liquid.
Flame-hydrolysed silica powder is relatively inexpensive and commercially available as Aerosil(trademark) silica powder from Degussa Limited with particles having a primary size in the range of 5-50 nm. Although flame-hydrolysed silica particles are initially hydrophilic, the surface chemistry is changed during step (d) by converting silanol functional groups (xe2x89xa1Sixe2x80x94OH) on the surfaces of the particles, to siloxane bonds (xe2x89xa1Sixe2x80x94Oxe2x80x94Sixe2x89xa1). This interaction has the advantage of rendering the particles hydrophobic through reaction with the PDMS or a surface modifier. A modified gel made in this way is particularly hydrophobic for two reasons. Firstly, the chemical properties of siloxane bonds make it inherently hydrophobic. Secondly, the small size of the particles in the flame-hydrolysed silica gives the modified gel a small-scale roughness which increases the hydrophobicity of the silicon dioxide.
Although silica and silica-based particles are preferred, other materials of hydrophobic character which can be prepared with a sufficiently small particle size could be used in conjunction with a hydrophobic gel capable of bonding to the particles. Examples include particles and gels formed from a metal oxide, such as titanium dioxide. For instance, titanium dioxide particles could be reacted with a titanium dioxide gel formed from titanium tetraisopropoxide.
A specific embodiment of the first and second aspects of the invention will now be described. In step (a) the precursors are 5 g of TEOS, 1.7 g of water acidified with HCl to a pH of approximately 4, and 2.7 g of ethanol. In step (b) the mixture is refluxed for 6-36 hours to form a silica gel. In step (c) the particulate material added to the gel is 0.5 g to 2.5 g of flame-hydrolysed silica powder, forming a mixture. About 0.5 g to 5 g of PDMS and approximately 50 g of isopropanol is then added to the mixture. In step (d), the reactants are mixed well and vibrated in an ultrasonic bath for 15 to 30 minutes, forming a uniform slurry. The slurry is then allowed to reflux for another 6-24 hours to make the silica gel chemically bond with the silica powder and PDMS. The slurry is the modified gel which can be used to form a coating according to the second aspect of the present invention. In a preferred embodiment, step (e) includes depositing drops of the slurry onto a substrate while the substrate is spinning. Typical substrates are metals, glasses, and ceramics. The substrate is spun at spin rate of 1000 to 2000 rpm. In step (f) the coated substrate is placed in an oven at a temperature of about 400xc2x0 C. for 10 to 30 minutes.
In a third aspect, the present invention provides a modified gel produced by the method according to the first aspect of the present invention.
In a fourth aspect, the present invention provides an object having a surface, at least a portion of which is coated with a hydrophobic coating formed from a modified gel made by the method according to the first aspect of the present invention.
In a fifth aspect, the present invention provides a hydrophobic coating produced by the method according to the second aspect of the present invention.
In a sixth aspect, the present invention provides an object having a surface, at least a portion of which is coated with a hydrophobic coating produced by the method according to the second aspect of the present invention.
In a seventh aspect, the present invention provides a use of the method according to the second aspect of the present invention to coat at least a portion of a surface of an object.
Surfaces which can be treated with the hydrophobic coating include metals, alloys, glasses, ceramics, composites, but can also include other materials. The surface treatment can be used to inhibit corrosion or the formation of crystallisation centres. The treatment can be used to prevent icing on surfaces, to produce anti-griping hydrophobic coatings for aboveground fixed facilities such as buildings and other structures, to provide anti-icing and anti-corrosion coatings for aircraft, or to provide anti-icing, anti-fouling and anti-corrosion coatings for maritime and inland waterway vessels. A hydrophobic anti-fouling coating formed from silica has the advantage that it much less toxic to the marine environment than most currently-available coatings.
Other uses include increasing the resistance of roofs to microflora colonisation. The hydrophobic coatings may also be used for extending the survivability, performance, and reliability of instruments and equipment.
Still other uses envisaged for the hydrophobic coatings include reducing drag in vessels such as canoes, yachts, ships, and other watercraft, improving the corrosion resistance of cooling systems in internal combustion engines utilising closed heat exchangers having liquid heat transfer agents, providing anti-corrosive and anti-icing coatings for undercarriages of vehicles such as tractors and combines and for agricultural machinery in general.
The hydrophobic coating according to the present invention may also be used to waterproof building foundations and structures and radioactive waste storage facilities, extend the operating service life of water cooling towers, protect railroad ties from microflora, provide anti-icing coatings for cooling chambers, refrigerators and chillers, extend the life cycle of hydroelectric power dams, and improve the efficiency of wind-driven motors. Metal parts coated in the hydrophobic coating would be less prone to rust because water would be repelled from the metal surface. The hydrophobic coating could also be used on windows, such as car windscreens, if the coating is largely transparent to visible light.
Throughout this specification, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.